Gas turbine engines are utilized for a large variety of purposes including propulsion by thrust, propulsion by mechanical coupling, driving accessories requiring a rotary input, providing compressed air, and combinations thereof. The compressed air provided is known as "bleed air" because it is bled from the turbine engine at some location following partial or total compression by a rotary or centrifugal compressor utilized in such engines. It may be utilized for a variety of purposes. For example, in an aircraft, it may be utilized for cabin ventilation, deicing, main engine starting, etc.
In any event, many of the uses to which bleed air is put are variable in the sense that quantity of bleed air required for a given use will vary over a period of time. At the same time, the demand for air to support combustion for operation of the turbine engine will remain essentially constant. As a consequence, a decrease in the demand for bleed air, without more, can result in so-called compressor surge or backflow that will occur because of the presence of a higher pressure in the combustor for the engine than in the diffuser for the combustor.
As is well known, this results in unstable operation of the turbine engine.
To avoid this problem, the prior art has resorted to the use of, for example, surge protection valves which are operable to open a flow path through which bleed air in excess of that demanded at a particular time may be dumped to prevent compressor surge. This method providing surge protection is satisfactory in preventing surge from occurring but requires that the turbine engine operate for a greater period of time at or near a full load condition. This relatively high loading on the engine reduces engine life and in addition, consumes unnecessarily large quantities of fuel.
The present invention is directed to overcoming one or more of the above problems.